@MathiasRenner there are some matrix clients that support voice messages already, fluffychat (https://christianpauly.gitlab.io/fluffychat-website/en/) for example. The protocol does support it, we just need more client developers to integrate it into their apps.. I think this is a place where community contributions would help quite a bit :)
@jcgruenhage Thanks so much for this info! Tested it -> works as expected. Nice!! I'll spread the word.